1. Field of the Invention
The present invention relates to a magnetoresistive effect element, and a magnetic head and a magnetic reproducing apparatus including the same.
2. Description of the Related Art
In recent years, the development of a magnetoresistive effect element has been progressing. In particular, in accordance with the development of a magnetoresistive effect film exhibiting a giant magnetoresistive effect (GMR), a magnetic device including the same, especially a magnetoresistive effect head (MR head) used as a magnetic head has made a remarkable improvement in its performance.
A spin-valve film is a known example of the GMR film. The spin-valve film has a nonmagnetic layer (called an intermediate layer, a spacer layer, or the like) interposed between two ferromagnetic layers. In such a structure, an exchange bias magnetic field is applied to one of the ferromagnetic layers (called a magnetization fixed layer, a pinned layer, or the like) so as to fix its magnetization, and the magnetization of the other ferromagnetic layer (called a magnetization free layer, a free layer, or the like) is inverted by an external magnetic field such as a signal magnetic field. Then, a relative angle of magnetization directions of the magnetization fixed layer and the magnetization free layer is changed, so that a high magnetoresistive effect can be obtained.
As a GMR element, there has been proposed an element having a so-called CPP (Current Perpendicular to Plane) structure in which a sense current is passed in a direction perpendicular to a film plane of a magnetoresistive effect film. In a spin-valve GMR element, an improvement in a MR change ratio by applying the CPP structure is also expected, and it has been reported that the CPP-GMR element has achieved a MR change ratio about ten times as large as that of a CIP (Current in Plane) GMR element. Further, the CPP-GMR element also has an advantage of having a lower resistance compared with that of a TMR element utilizing a tunnel effect.
In the spin-valve GMR element, the total thickness of spin-dependent layers is very small and the number of interfaces is also small. Therefore, when the CPP structure is applied thereto, the resistance of the element becomes small, resulting in a small output absolute value. For example, when a current is perpendicularly passed to a spin-valve GMR element in which the thickness of each of a magnetization fixed layer and a magnetization free layer is 5 nm, an output absolute value is as small as about 0.5 mΩ per 1 μm2. That is, in order to put the spin valve GMR element with the CPP structure into practical use, it is important to heighten a resistance value of a portion involved in spin-dependent conductance in the element to increase a resistance change amount, thereby making its output larger.
Regarding this point, a known method for improving a magnetoresistive effect is a method in which a resistance adjusting layer including an insulator is inserted as an intermediate layer in a spin-valve film (see J. Appl. Phys. 89, 6943 (2001), IEEE Trans. Mag. 38, 2777 (2000)). A spin-valve GMR element has portions where electrons make spin-dependent scattering (a magnetization fixed layer/an intermediate layer/a magnetization free layer) and portions with small spin-dependent scattering (a base layer, an antiferromagnetic layer, a protective layer, and so on). A magnetoresistive effect of the spin-valve GMR element (MR) can be expressed as MR=Rsd/(Rsd+Rsi) where Rsd is a resistance of the former and Rsi is a resistance of the latter. Therefore, MR is more enhanced as Rsd is larger than Rsi to a greater extent. The spin-valve GMR element with the CPP structure having the resistance adjusting layer including the insulator utilizes this effect.
In the spin-valve GMR element with the CPP structure, the resistance adjusting layer is composed of a complete insulator part where no current flows and a conduction part (metal path) allowing the current flow. In the vicinity of the resistance adjusting layer, the current is constricted toward the conduction part. In the spin-valve GMR element with the CPP structure as described above, owing to the current constriction effect, portions right above and right under the conduction part serve as current paths both in the magnetization fixed layer and the magnetization free layer. Therefore, controlling the state of the current paths including the conduction part is important in order to improve the magnetoresistive effect (MR). However, the conventional spin-valve GMR element only increases the resistance of the spin-dependent scattering part by the resistance adjusting layer (intermediate layer), and no control over the state of the current path is performed. Therefore, a large magnetoresistance effect (MR) owing to the adoption of the conduction part has not been achieved yet with good reproducibility.